Corticosteriods (CS) remains the mainstay of therapy for pediatric UC; however, 20% of patients fail to respond to CS therapy and require secondary-line therapies and escalation of medical management or colectomy4. In recent patient-based studies, we demonstrated that CCL11 levels correlated with tissue eosinophil numbers, which, in turn, correlated with the UC Histologic Index of Severity (UCHIS) in pediatric UC. In preliminary studies, we determined that high rectosigmoid eosinophil levels at the time of pediatric UC diagnosis is linked with reduced likelihood of achieving steroid-free remission (SFR). Notably, high eosinophil levels positively correlated with levels of CCL11 and a low molecular weight intracellular calcium binding protein called calprotectin. In experimental studies, we have identified a link between calprotectin, M-derived CCL11, and eosinophils in experimental colitis. Importantly, we show that colonic Ms express the calprotectin receptor (RAGE); and calprotectin stimulates p65 activation and CCL11 expression in Ms in vitro. The object of this application is to further understand the relative contribution of calprotectin to inflammatory M-derived CCL11 and eosinophilic inflammation in experimental colitis and pediatric UC and treatment responses. Our central hypothesis is that calprotectin-induced activation of Ms regulates CCL11-dependent colonic eosinophilic inflammation in experimental colitis and pediatric UC and that this pathway drives refractory disease. We will test this hypothesis by examining the relationship between calprotectin, Ms, CCL11, and eosinophils in pediatric UC at diagnosis and the relationship of this pathway to refractory disease. We will employ S100A9-/-, RAGE-/-, CCR2-/-, CCR2-/-/Il10-/- mice; use DSS- and NSAID-induced Il10-/- models of colitis; generate bone marrow-derived M-specific chimeric mice and perform eosinophil chemotaxis assays to define M-derived CCL11 in eosinophil recruitment and histopathology in experimental colitis and the sensitivity of this pathway to CS treatment. With respect to expected outcomes, the studies proposed in Aim I are expected to define the association between calprotecin, CCL11+ Ms, and eosinophils in pediatric UC; the sensitivity of this pathway to CS; and the predictive value of CCL11 and eosinophils disease as an indicator of resistance to first-line therapies. Aim II is expected to delineate colonic eosinophilc inflammation and histopathology dependency on calprotectin/RAGE-induced M-derived CCL11 in experimental colitis. Aim III is expected to define the capacity of calprotectin/RAGE to induce M-derived CCL11 and eosinophil chemotaxis and sensitivity to CS-induced inhibition. Demonstration of calprotectin/M/CCL11/eosinophil axis involvement in resistance to first-line therapies in pediatric UC will have important clinical implications for both the usage of CCL11 and eosinophils as a prognostic indicator for resistance to first-line therapies at diagnosis and the possible usage of therapeutic agents targeting eosinophils and eosinophil regulatory molecules as an approach for improved treatment of pediatric UC and refractory UC.